In general, a spark plug used for an internal combustion engine such as an automotive engine includes a center electrode disposed in a combustion chamber of the internal combustion engine, and a ground electrode disposed to face the center electrode via a spark discharge gap. Such a spark plug produces spark discharge at the spark discharge gap within the combustion chamber of the internal combustion engine to thereby burn an air-fuel mixture charged into the combustion chamber.
When an internal combustion engine to which such a spark plug is attached is started in a low temperature environment, or when the internal combustion engine to which such a spark plug is attached is of a direct injection type, fuel that is injected into a combustion chamber may hit directly against an ignition portion of the spark plug, whereby the fuel may adhere to and remain between the center electrode and the ground electrode in the form of a droplet, to thereby form a so-called “fuel bridge.” If a fuel bridge is formed between the center electrode and the ground electrode, spark discharge fails to be properly generated between the center electrode and the ground electrode, and the ability of the internal combustion engine to start is reduced greatly.
In order to solve such a problem caused by a fuel bridge, there has been proposed a spark plug having a larger spark discharge gap between a center electrode and a ground electrode thereof. For example, Patent Document 1, Japanese Patent Application Laid Open (kokai) No. 2007-250258, describes a spark plug for an internal combustion engine which comprises a mounting bracket having a mounting screw portion provided on the outer circumference thereof. An insulator is held inside the mounting bracket, and a center electrode is held in an insulator hole of the insulator. A ground electrode forms a spark discharge gap in cooperation with the center electrode. As viewed from the front end side of the spark plug, the area S1 of a portion of the insulator hole located outside the outer edge of the ground electrode and the area S2 of the entire insulator hole have a relation S1/S2≦0.3. In addition, the projection amount L of the center electrode from a front end portion of the insulator is equal to or less than 0.6 mm; and the minimum and maximum values Hmin and Hmax of the distance between a flat surface formed on the insulator front end portion and a flat surface formed on the ground electrode and facing the former flat surface have a relation Hmax/Hmin≦1.3. Still further, the thickness T of the insulator between the insulator hole and the outer circumferential surface of the insulator is equal to or less than 0.7 mm; and the diameter d of the front end portion of the center electrode is equal to or less than 0.6 mm.
Incidentally, when the spark discharge gap of a spark plug is enlarged, the discharge voltage at which spark discharge occurs tends to increase. Therefore, the capacity of a coil power source imposes a certain limit on expansion of the spark discharge gap.
In the case where a spark plug is of a so-called “parallel type” in which a distal end portion of the ground electrode is disposed on the axis of the center electrode to face the end surface of the center electrode as in a spark plug described in Patent Document 1, Japanese Patent Application Laid Open (kokai) No. 2007-250258, even when the spark discharge gap is enlarged within a range in which the above-described characteristic of the spark plug can be maintained, a fuel bridge is apt to be formed and the formed bridge is apt to be maintained, because the spark plug has a bent ground electrode. Therefore, the parallel-type spark plug may fail to completely solve the above-mentioned problem of degraded startability.
Meanwhile, when starting and stopping of an internal combustion engine is repeated or short-time operation of the engine is repeated in a low temperature environment, a phenomenon in which carbon adheres to the surface of the insulator of a spark plug (hereinafter may be referred to as “sooting up”) is likely to occur, which may lower insulating performance and igniting performance. Accordingly, an internal combustion engine, in particular, an internal combustion engine used in a low temperature environment is desired to have a high sooting-up prevention performance.